Vertical vessels that are skirt-supported typically have access openings in the skirt. These openings need to be analyzed, especially when they are of reasonable size, on tall towers, or under sizable wind/seismic loads. The information below explains skirt openings input in PV Elite.
Entering the Data
Users sometimes ask, "How do we enter the information?" Well, first indicate that the skirt element has the opening(s). Check the box to indicate this, as shown in the screenshot below:
This will open the following dialog. Consider the various features of the data entry table. It looks like this:
Consider the various components of the table.
Number of Skirt Openings (0-10)
From the dropdown list box, choose the number of holes in the skirt. There can be up to 10 holes.
Width, Height, Center Spacing, Frame Thickness, Frame Width
Here are the dimensions:
The layout angle is the location (in plan view) around the skirt:
Pitch and Ligament
In the skirt input, two very important dimensions are mentioned, and please make note of them. They are the pitch and the ligament. Here are those dimensions:
The pitch is the center-to-center distance between adjacent holes. The ligament distance is very important. If the ligament is too small, there is a possibility of collapse and buckling, which is a very dangerous situation. EN 13445, the code chosen as the basis for the analysis, is silent on this issue. This is because EN 13445 only gives consideration to one hole, not the multiple holes that may be analyzed by PV Elite.
The output processor (results) gives a recommended minimum ligament distance, but the user must evaluate whether this meets project requirements. The following image shows a sample of the results.
Vessels with Multiple Skirt Elements
Vessels with multiple sirts raise some interesting questions.
Analyzing access openings and other holes in skirts can be accomplished by observing a few precautionary measures. Complications can arise when there is more than one skirt supporting the vessel. Figure 1 illustrates this:
The bottom of the obround opening breaks the bottom of the skirt, but PV Elite merely issues a note to that effect, but performs the analysis without any problems.